HV and LV fuse discrimination on transformer

[Cerkit]

Hi,

I am looking at the fuse characteristic graph for an HV and LV fuse to assess suitable discrimination between the two. I am a bit unsure as to what the impact of changing the voltage ratio of the transformer is when the impedance of the transformer stays the same.

I have attached the graph as an example. If I was looking at 100kVA transformer with 5% impedance and a source fault level of 150MVA. Would the transformer voltage ratio play any role in the discrimination assessment? If system was a 11000/440V versus a 6600/440V? I don't see why it should as the maximum fault level through the transformer should still be the same.

Thanks

 

[RRaghunath]

For the same kVA rating, the rated current at 6.6kV and the rated current at 11kV will be different and thus the HV fuse rating will be different.
Though the through fault current is same, the current multiples as seen by the HV fuse are lower if it were 6.6kV which will affect its operating time.

 

[7anoter4]

There¯are¯not¯enough¯data.See:

http://electro-sistem.com/wp-conten...ormer-protection-with-fuses-2017_10_19-EN.pdf

 

[Cerkit]

Hi,

I don't think I understand exactly how it works. Please see the link below

https://electrical-engineering-portal.com/fuses-protecting-rural-distribution-systems

They mention that in the case of transformer HV and LV fuses, the current values must be converted to a common voltage base.
For the same configuration mentioned for figure 3 in the link above (100kVA transformer with 4.75% impedance), if the transformer was a 6.6/0.44kV ratio instead of 11/0.44kV can I still use the graphs to determine the minimum current at which the 400A LV fuse will grade with the 25A HV fuse?

Thanks

 

[stevenal]

If your dashed TCC was correct for 11 kV, shift it to the left by a factor 6.6/11 to see the new result. Looks like you were miscoordinated to begin with.